Download CE Amplifier Frequency Response

Lab 2
Common Emitter Amplifier
Frequency Response
November 6, 2014
Michelle Acosta
Executive Summary:
The Common Emitter Amplifier is a single-stage amplifier that is most often used
as a voltage amplifier. The Bipolar Junction Transistor (BJT) is configured with the input
at the base, the output at the collector, and the emitter common to both or, in this case,
grounded.
In
order
to
study
the
CE
Parameter
Requirement
Amplifier and its frequency response,
Midband Gain
AM ≥ 125V/V
parameters were set in order to give a
Low-Frequency Response
L

High-Frequency Response
H
≥
basis for a well working voltage
amplifier. By selecting an input current
and four resistor values, the amplifier
met the criteria annotated in Table 1.
DC Power
PDC  35mW
Table 1: Design specifications of a Common Emitter
Amplifier were given.
Lab Procedures and Measurement Discussion:
The
lab
consisted
of
hand
calculations, PSPICE simulation, and
measurements of the circuit shown in
Figure 1.The hand calculations served
as
a
basis for
connecting
lecture
material to the lab setting. The PSPICE
simulation was a quick and easy way to
confirm that specifications were met.
Finally, the measurements gave a real
world example of the CE Amplifier that
was designed according to the given
specifications. The circuit was measured
for DC and AC values and for values to
plot a frequency response. In the end,
Figure 1: The Common Emitter Amplifier is most
often used as a voltage amplifier. The resulting gain
of the CE Amplifier should be greater than 125 V/V
while using less than 35 mW of DC power.
the specifications were met and the
frequency response was plotted.
2
Being that the most difficult part of the lab was finding a starting point, the lab TA
gave vital hints. The initial current that was used to calculate values by hand was
IC=1mA. However, the specifications of the frequency responses were not met with this
current so another had to be chosen.
The next value attempted was IC=1.8mA and the specifications were met.
However, although the specifications were met, there may have been a better design
that would produce a higher gain. Deciding that reworking hand calculations would be
too time consuming, a Matlab code was written in order to efficiently calculate the
specified values. In the end, it was decided that I C=1.5mA would be best fit for the
design.
As stated previously, the main focus of the lab was to study the frequency
response of the CE amplifier. The frequency response of the CE amplifier would ideally
look like that of the plot in Figure 2(a). The area where the plot is generally flat is the
midband gain (AM) and it is in this area that the gain is unaffected by all capacitances.
The area to the left of the midband gain is where the gain is affected by the base and
emitter capacitances, Cb and Ce. The area to the right of the midband gain is where the
gain is affected by the internal capacitances Cμ and CThe points at which the
frequency response intersects the 3dB frequency are the areas of interest as these
correspond to two of the specifications wL and wH.
(a)
(b)
Figure 2: Figure 2(a) depicts the ideal frequency response of the CE amplifier designed in the
lab. Figure 2(b) depicts the measured frequency response of the CE amplifier. The values for
the frequency response were obtained by varying the frequency and measuring the voltage.
3
At low frequencies, the capacitances Cb and Ce cannot be ignored. Unfortunately,
there is not just one low-frequency response. In order to calculate fL, the superposition
principle must be used. The effects of each capacitor should be analyzed by shorting
the other capacitor. The two values can then be added together to give the approximate
value of the low-frequency 3dB frequency, assuming that the two capacitors are no
interacting.
On the other hand, at high frequencies, Cb and Ce have very small impedances
and can therefore be replaced by short circuits. At the midband frequencies, open
circuits can replace the internal capacitors of the BJT. However at high frequencies, the
effects of both Cμ and Cmust be accounted for. Both internal capacitances are given
by the datasheet making the high frequency 3dB frequency relatively easy to calculate.
The voltage at which the wL and wH frequencies occur at of the physical circuit
was measured to be 1.4V. From there voltage values were measured by varying the
input frequency. At this point the frequency response of the designed circuit was plotted
using excel. Figure 2(b) depicts the measured frequency response and it can be seen
that it is similar to the frequency response in Figure 2(a).
Conclusion:
The Common Emitter amplifier was studied in this lab. The design specifications
were met and the frequency response was obtained. It was seen that the there was a
large midband gain with low and high 3dB frequencies making the CE amplifier an
excellent choice for a voltage amplifier.
4
Summary Table
5
Hand Calculations
6
PSPICE Frequency Response
PSPICE Final Circuit
7